THURSDAY, Aug. 15 (HealthDay News) -- Hold the java and listen up: A new study suggests that people under 55 who drink four or more cups of coffee a day are more likely to die early than others.

Skeptical experts cautioned that the study of over 40,000 adults has major limitations and the findings don't show that coffee actually causes an early death. But study co-author Dr. Carl Lavie, professor of medicine at the John Ochsner Heart and Vascular Institute in New Orleans, believes there's reason to be worried if you down a lot of joe.

"Small amounts of coffee, up to 28 cups a week, seem to be safe," he said. "But there's a reason to try to keep your intake at below four 8-ounce cups per day, particularly for younger people."

At issue is the hotly debated topic of whether coffee is good for you, and if it is, how much is too much? In recent years, scientists have pored over existing research and found signs that coffee drinking -- even heavier consumption -- actually reduces the risk of cancer, heart failure and other conditions.

It's hard to know for sure, however. It would be difficult to launch the most rigorous type of research in which scientists would assign some people to drink large amounts of coffee and have others avoid it. It's also possible that coffee drinkers may share something other than a taste for brewed beans that affects their health. Hyperactive personalities? Trouble staying awake? Money to spend on hot drinks?

For a Starbucks-studded nation, this is not encouraging news. In the United States, more than 60 percent of adults report drinking coffee daily, consuming just over three cups a day on average, according to the National Coffee Association.

For the new study, published Aug. 15 in Mayo Clinic Proceedings, researchers tracked nearly 44,000 people aged 20 to 87 for an average of about 16 years. About three-quarters were men, and they entered the study between 1971 and 2002.

During the follow-up period, 2,512 people died from all causes. After adjusting their statistics for factors such as smoking and fitness, the researchers found that those who drank more than 28 cups of coffee a week were 21 percent more likely to have died. The risk was 50 percent higher in men and women younger than 55.

Lower levels of coffee-drinking didn't seem to affect mortality. "We were honestly hoping to find that low amounts of coffee were beneficial," said Lavie, "and we were hoping no dose was toxic."

Many questions remain unanswered, however, and the study authors said further research is needed. Lavie said it's possible that coffee might contribute to cancer, but researchers can't confirm that unless they dig deeper to see what killed participants in the study. It's also possible that certain genetic factors put heavy coffee drinkers at greater risk, the study suggests.

Rob van Dam, an associate professor at the National University of Singapore, said the study has significant shortcomings. Among other things, he said, it didn't examine dietary habits or look at causes of death in depth. "There have been several previous studies on this topic, and recommendations about coffee consumption should consider the totality of the evidence rather than only the results from a single study," he said.

Rachel Huxley, chair of epidemiology at the University of Queensland in Australia, cautioned that smoking -- much more common among heavy coffee drinkers than others -- may have thrown off the results by contributing to death rates. The study results don't do enough to take this into account, she said.

Huxley added that previous research as a whole suggests that the level of coffee consumption in question actually helps people live longer.